1. Field of the Invention
The present invention relates to a method of obtaining dried terephthalic acid by removing acetic acid and/or water from a slurry containing terephthalic acid, and acetic acid and/or water.
2. State of the Art
Industrial production of terephthalic acid generally employs a process of oxidizing paraxylene, the raw material, in the presence of a salt or salts of heavy metal(s) such as Co or Co+Mn as a catalyst, with molecular oxygen, in liquid medium of lower aliphatic acid, usually acetic acid.
There are a few types of this process using different reaction conditions: some of them provide, after the oxidation step, purification step by hydrogenation in water medium to prepare terephthalic acid of a desired purity; and the others offer, in case where high purity is not required, produced terephthalic acid without purification step.
Anyway, in industrial production of terephthalic acid, it is essential to provide a step of recovering terephthalic acid as a dry powdery material by separating it from acetic acid and/or water used as solvent.
Heretofore, there has been employed a manner of drying terephthalic acid, after draining off in centrifugal separator, to heat and dry it with steam in a rotary drier in the stream of non-condensing gas such as nitrogen. In this manner, it is possible to reduce residual liquid in the terephthalic acid after the drying to about 0.1% by weight.
However, rate of drying in this manner is low because the rate depends on vapor pressure of acetic acid and/or water at the heating temperature. Therefore, there are many problems in the drying: long staying period in the dryer, which requires a large apparatus, decreased heat transfer effect due to adhesion of terephthalic acid on wall of tubes in which heating medium (steam) passes, and resulting troublesome maintenance. Moreover, the drying requires system of recycling gas for the drying such as nitrogen, namely, a blower for the recycling and attached apparatus such as scrubber for recovering entrained terephthalic acid and a heat exchanger for heating the dryer gas. Also, as to the operation of the drying system, it is necessary to maintain a constant rate of draining-off in the centrifugal separator in order to keep smooth feeding of the slurry to the rotary dryer with a screw feeder. If residual liquid content varies, terephthalic acid to be fed becomes solid masses in the screw feeder, which prevents transfer and even forces interruption of operation of the dryer system. Further, perfect gas seal in the rotary dryer is difficult to realize and the dryer gas inevitably leaks.